More specifically, the invention relates to a compound web of two plies of absorbent paper made of conventional creped cellulose wadding and having a specific surface weight of 10 to 40 g/c.sup.2. The plies are embossed by calendering into fine patterns of protrusions and are bonded to each other, especially by glueing, at the surfaces of the peaks of the coinciding protrusions of the two plies.
U.S. Pat. No. 3,414,459 describes a method for embossing and bonding to each other two cellulose wadding plies using the so-called tip-to-tip technique. In this technique, the two plies are embossed separately using identical metal cylinders fitted with embossing protrusions and, respectively, cooperating with rubber-clad rollers. Next, a suitable glue is deposited on the protusion peaks of one of the plies. Lastly, the two plies are bonded to each other at the peaks of the protrusions by being clamped between the two embossing cylinders which are rotatably ganged to each other in such a manner that the embossing protrusions meet tip-to-tip. This technique entails that the two cylinders include symmetrical patterns and that the embossing protrusions of the two cylinders perfectly coincide inside the clamping gap. Radial or circumferential slippage between the embossing protrusions of the two cylinders can lead to a lack of adhesion in some zones of the compound web so made.
To remedy these drawbacks, U.S. Pat. No. 5,173,351 proposes using patterns of different repeats, in at least one direction, on the two embossing cylinders to ensure that there is at least one bonding site on the elementary compound webs produced by cutting the compound web issuing from the equipment.
Lastly, WO 96/32248 patent application suggests bonding only some zones of the two embossed plies by applying glue only to the ends of the protrusions of one of the plies taking part in the actual bonding. In this design as well, the two cylinders must have symmetrical patterns and the embossing protrusions of the two cylinders must accurately coincide in the clamping gap.
In the designs of these three documents, the embossing protrusions are mounted on the cylinders along generatrices of regularly spaced helices and great circles. Consequently, the protrusions formed in the plies by the embossing protrusion imprints are aligned in preferential directions and at constant distances. The observer viewing the outsides of the compound web is then under the impression that these surfaces are fitted with geometric designs solely constituted of straight lines or straight line segments.
It is clear that a slight axial or circumferential shift between the two cylinders caused by a lack of synchronization or by wear of the drive means can lead to a lack of coincidence between the embossing protrusions within the clamping gap of the embossing cylinders and thus shall entail rejects.
Other techniques for assembling the plies allow nesting of the protrusions of one of the plies between the protrusions of the other ply so as to achieve a so-called nested structure which offers improved absorption. Using such a technique, the embossing cylinders are moved apart and the embossed ply is removed from one of the cylinders to nest the ply in the other ply by passing in a clamping gap between the other embossing cylinder and a smooth complementary cylinder. This technique also requires perfect synchronization of the two embossing cylinders.